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PAYG credit is non refundable, you should really have used the credit or donated the credit to a charity before changing to pay monthy. As an encouragement to change to pay monthly Vodafone used to give any remaining PAYG credit as a credit on your first pay monthly bill but quite sure that has been stopped.
However, it won't do any harm speaking to Vodafone and asking if there is any way to do a goodwill and if the credit can be credited to your pay monthly account, at least this will make the first bill easier. I would suggest the Social Media Team here: Contact
Good luck but no promises.
Hi lckfiz1990! When you move from Pay As You Go to a Pay Monthly contract, andy credit left on your account is lost, so we wouldn't be able to refund this for you. If you would like to discuss this further, I'd recommend popping us a message over Social Media here
Not quite sure where the goodwill that Vodafone were known for providing to customers had disappeared to.
I would recommend following @Jack 's advise and post this on Twitter and Facebook, it's social media and public. Vodafone are keeping your remaining credit as their own profit, it would be different if it was going to a charity.